The present invention relates to an apparatus for measuring the velocity of flow of fluids, particularly the intake air of internal-combustion engines, having an electrically heatable resistance layer which is applied in insulated manner on a support and is in thermal contact with the flowing fluid.
In known sensor elements for electrothermal air mass meters, a resistance layer which is applied to a flat support is passed through by a current, resulting in heating. Depending on the velocity of flow of the air flowing around the sensor, a greater or lesser amount of heat is removed from the resistance layer and the sensor. By the use of a resistance material with positive or negative temperature coefficient, the current can be controlled in such a manner that the temperature is a constant amount greater than the temperature of the air. The current required for this is then a measure of the velocity of flow.
One important field of use of such sensors is for the measurement of the mass of air drawn in by internal combustion engines. This measurement is particularly important in order to control the combustion process of the internal-combustion engine so as to minimize the proportion of injurious substances upon the combustion.
However, it has been found that the local heat transfer coefficient is greatest in the vicinity of that edge of the sensor which is opposite the flow of air, which coefficient decreases with increasing distance from said edge. In this way a temperature gradient is produced in the direction of flow along the resistance layer and the support, said gradient being dependent on the velocity of flow and causing a relatively slow response time of the known sensors.
In order to reduce the response time it is already known to dimension the current paths produced in the resistance layer in such a manner that the resultant electric heating output per unit of area is in a constant relationship to the local heat transfer coefficient.